The instant invention relates to a process for bonding steel sheet together, as well as manifolds and other products made using the same. More specifically, the instant invention relates a process for bonding steel sheet together, at relatively low temperatures (circa 1900xc2x0 F., 1037.8xc2x0 C.), without using intermediate bonding agents.
In the art of gas and/or liquid chromatograph devices, channels are used to connect different flow components. For example, a channel may be used to connect a valve to a regulator, or to connect a valve to a pressure sensor, or to connect a valve to a detector. These channels can be made from tubes and fittings. Preferably, however, these channels are etched in the interior of a manifold. As used herein, the term xe2x80x9cmanifoldxe2x80x9d means any construct of two or more metal sheets that contain one or more channels etched into its interior.
The channels are generally formed by etching mirror image metal sheets. The metal sheets are then contacted and bonded to form the manifold. The etched surfaces on the metal sheets, when combined, form the channels. For gas chromatograph applications, it is extremely important to create a strong leak free bond between the metal sheets. Otherwise, the channels may burst due to pressure exerted by chemicals run through the channels.
Conventionally, the channels are made by etching and contacting two pieces of stainless steel sheet (generally 316L). Transient liquid phase (TLP) diffusion is then used to create a leak proof bond. TLP diffusion bonding requires plating a layer of sulfamate nickel followed by a layer of electroless nickel (11%xc2x11% phosphorous) on the surface to be bonded. The plated layers act like a glue and permit bonding at relatively low temperatures, circa 1800xc2x0 F. (982.2xc2x0 C.). In contrast, bonding by fusion requires temperatures at or near the melting point of the stainless steel sheets.
However, the plating process is manually operated and difficult to control. A number of parameters, including plate composition, homogeneity, uniformity, and thickness, affect the nature of the bond ultimately obtained. Furthermore, it is difficult to adequately measure and, therefore, monitor plating. For example, Glow Discharge Spectroscopy is capable of measuring the depth composition profile of a plated layer but it is extremely expensive and needs absolute standards for instrument calibration.
It would be desirable to eliminate the need for TLP diffusion bonding without losing the ability to generate low temperature bonds. Unfortunately, until the instant invention, it was believed that solid state diffusion bonds would not form between like materials absent temperatures near the material""s melting point (even under load). The melting point of steel is extremely high. For example, 316L stainless steel sheet melts at around 2540-2630xc2x0 F. (1379.4-1428.9xc2x0 C.). Operating at such high temperatures requires special equipment and is expensive.
In the instant invention, multiple steel sheets are bonded at temperatures as low as 1700xc2x0 F. (926.6xc2x0 C.) without using the TLP diffusion bonding process. The invention eliminates the need for the plating step in TLP diffusion bonding, and the troubles associated with the same. Accordingly, it is simpler and more reliable. The instant invention generates a leak tight bond with high bond strength without affecting the properties of the steel.
The invention bonds the steel sheet through the following steps: (i) contacting two or more steel sheets; (ii) heating the contacted sheets to a temperature substantially below their melting temperatures; (iii) applying pressure to the contacted sheets; and (iv) cooling the contacted sheets. Products, such as manifolds, made in accordance with the invention, have an interface between the bound sheets, no intermediate foreign bonding material, and grains that grow from one sheet across the interface into the other sheet.
The peak temperatures employed in the invention are at least about 1700xc2x0 F. and at least about 200xc2x0 F. below the melting temperature of the steel. Accordingly, this is a low temperature process that, like the TLP diffusion bonding process, does not require great expense or specialized equipment.